Hitherto, various technologies of protecting a rider by an airbag device mounted to a motorcycle have been known. For example, a technology used in a motor-bicycle for restraining a rider by deploying and inflating an airbag, accommodated in a case mounted to a vehicle frame, by inflation gas when the motor-bicycle collides at a location which is situated in front of the motor-bicycle is known, refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-137777. In this technology, it is possible to provide a wide area of protection using the airbag. However, there is a strong demand for a technology which is more effective in reliably restraining a rider by an airbag when an airbag device is mounted to a vehicle body having an open structure in all directions like the body of a motorcycle.